Question

This function takes two integers and returns the list of all integers in the range [a,b]

This is the solution that I wrote.

let rec range_rec l a b = 
  if (a=b) then l@[b]
  else range_rec (l@[a], a+1, b);;

let range a b = range_rec [] a b;;

I’m hitting an error “Error: This expression has type int list * int * int but an expression was expected of type int”. Can someone throw some light on why am I getting this error?

Thanks.

Answer 1

It should look like this:

let rec range_rec l a b = 
  if a = b then l @ [b]
  else range_rec (l @ [a]) (a + 1) b;;

let range a b = range_rec [] a b;;

What I’ve done:

1. Changed loop to range_rec
2. Changed (l@[a], a+1, b) to (l @ [a]) (a + 1) b. The first is a triplet and the second is 3 arguments to a curried function.
3. Notice that if (a = b) then can be written as if a = b then.

Last, the function can be made more efficient by using :: instead of @ by looping “backwards”. For example like gasche have shown.